There are numerous laboratory systems and medical as well as pharmaceutical appliances which require precise pipetting operations in order to obtain satisfactory analytic precision. It is necessary for this purpose to precisely define the filling level in test tubes, titre plates and other liquid containers. There are also applications which are concerned with the detection of foam-liquid phase boundaries and gas-foam phase boundaries. The term phase boundary shall be used below both for transitions between gaseous and liquid media (gas-liquid phase boundary), for gas-foam phase boundaries, and also for transitions between different liquid media (foam-liquid phase boundary).
Such a determination of the phase boundary is particularly relevant concerning the automation of measurement or test sequences. The so-called determination of the filling level typically occurs by means of a detection of the liquid level, i.e. the position of the phase boundary between the air and the liquid is determined. This process is also known as “Liquid Level Detection” (LLD).
Various methods for determining the filling level are known from the prior art, which are based on different physical principles such as the detection of the light reflected by the surface of the liquid, or the measurement of electrical properties of the pipettes when they are brought into contact with the liquid. Since a gas and a liquid have distinctly different dielectric constants, the gas-liquid phase boundary can also be determined via a change in capacitance.
Liquid level detection is used in pipetting devices for example. In this case, the pipetting needle shall be immersed as little as possible into the liquid to be pipetted during suction with a pipette in order to keep contamination of the sample liquid as low as possible. During suction, the pipetting needle is therefore typically immersed only a few millimeters below the liquid level. It needs to be ensured however that the pipetting needle is immersed to a sufficiently far extent so that no air can be aspirated. During the suction process, the pipetting needle is then continuously moved along the decreasing liquid level so that it remains immersed to an equally deep extent in relation to the liquid level. After the suction, it can be calculated where the level of the gas-liquid phase boundary should be situated on the basis of the aspirated volume and the cross-sectional surface area of the liquid container. During the surfacing of the pipetting tip, a surfacing signal with the calculated position of the gas-liquid phase boundary can be compared in order to thus verify the pipetting process.
It is therefore desirable on the one hand to enable the positioning of the pipetting tip slightly beneath the liquid surface. On the other hand, the filling level can vary strongly from one liquid container to another, which is why the pipetting tip must be precisely positionable in large areas. It is therefore exceptionally important to enable the correct and definite detection of the liquid surface.
The reliability of the recognition of the liquid surface with the known methods is unsatisfactory in a number of cases, especially in the case of liquids which are susceptible to the formation of foam.
It is therefore important to enable distinguishing between foam and/or liquid contacting of a probe that can be advanced in a container (e.g. in form of a pipette).
Applications can also be considered in which only the detection of a foam boundary is concerned.